The electroplating of chromium is a well known concept which has been employed with varying degrees of success for the past several decades. The earliest processes for effecting this end involved the use of hexavalent chromium salts which yielded excellent wear resistance and proved to be suitable in numerous applications. However, workers in the art soon discovered that chromium plating with hexavalent chromium often and repeatedly led to the need to maintain high concentrations of chromium metal in solution.
Typically, such processes involve the use of approximately 250 g/l of hexavalent chromium and with the passage of plating time the trivalent chromium concentration increases to a level which has proven to be unsatisfactory for continued chromium plating. At that juncture, the failure to reduce the trivalent chromium concentration will result in deposits of poor quality.
Heretofore, the conventional method for the regeneration of a spent chromium plating bath having an unacceptable level of trivalent chromium involves the use cationic exchange resins wherein the spent chromium plating bath undergoes oxidation of trivalent chromium to hexavalent chromium. A common and cumbersome technique for effecting this end involves the use of a suitable tank which is used to form a reservoir of spent chromium plating solution. Anode assemblies are supported on the tank within the reservoir. Each of the anode assemblies includes two frame sections bolted together along their sides and bottom with separate cation selective membranes being secured between them. Membrane tensioners are then inserted between the membranes to separate them and urge them against the frame sections over openings therein to define an anode chamber. A suitable anode and cathode is then provided to generate a current flow therebetween which oxidizes the trivalent chromium in the depleted chromium solution in the anode chamber to form hexavalent chromium. The resultant solution has a lesser density than the depleted chromium solution and rises above the depleted chromium solution in the anode chamber. Then, regenerated solution is then drained from the top of the anode chamber and recycled for further use.
Still another technique for effecting this end, commonly referred to as the "Pfaudler Electrolytic Purification (EP) Cell" involves the use of ceramic cells to effect regeneration of the chromic acid solution which is circulated continuously through the EP cell from the plating tank. The EP chosen comprises cylindrical, unglazed ceramic cells holding a cathode and an electrolyte disposed intermediate a pair of external anodes. The electrodes chosen are typically lead which requires regular cleansing. The size of the tank chosen for this process and the number of cells required is determined by the size of the tank and the level of contamination to be removed.
Unfortunately, each of these prior art processes for regenerating chromium plating baths has been found unsatisfactory due to the fact that both processes result in the formation of a concentrated chromium sludge at the bottom of the process tank, such sludge containing a high hexavalent chromium content which must be treated and disposed of as a hazardous waste. This process is economically non-feasible since the hexavalent chromium ion is an environmental toxin and carcinogen.
Furthermore, ion exchange technique has a limited lifetime and can only be used a few times prior to replacement, another limiting economic factor. Membrane filters in the ion exchange resin frequently become contaminated with chromic acid and require frequent scrubbing, an extremely burdensome and hazardous operation which exposes workers in the art top toxic hexavalent chromium. Accordingly, the replacement of spent ion exchange resin columns adds still further economic burdens to the process. Accordingly, efforts to develop new and economically attractive techniques for effecting this end have been continuously pursued by workers in the art.